1. Field of the Invention
The present invention is directed to a device for operating a high pressure gas discharge lamp with the use of an electronic ballast having an improved lamp factor of substantially 1.0.
2. Description of the Prior Art
Electronic ballasts are expected to operate a high pressure gas discharge lamp for the reason that it can be made compact and lightweight as compared to a conventional so-called inductive ballast. In addition, the electronic ballast is known to be advantageous over the conventional inductive ballast in that the electronic ballast having a lamp power factor of substantially 1.0 requires less lamp voltage and lamp current for obtaining a rated operating wattage than the conventional ballast having lamp power factor of 0.9 or less, as is apparent from graphs of FIGS. 12A and 12B in which load characteristics of the lamp operated by the electronic ballast is designated by solid lines, while load characteristics of the lamp operated by the conventional ballast is designated by dotted lines. FIG. 12A shows the respective load characteristics with the lamp wattage W.sub.LA on the axis of ordinate and the lamp voltage V.sub.LA on the axis of abscissa. FIG. 12B shows the same load characteristics with the lamp voltage V.sub.LA on the axis of ordinate and the lamp current I.sub.LA on the axis of abscissa. As seen from the figures, the lamp operated by the electronic ballast requires less rated lamp voltage V.sub.01 and current I.sub.01 for producing rated lamp wattage W.sub.0 than rated lamp voltage V.sub.02 and current I.sub.02 for the lamp operated by the conventional ballast. In this respect, the electronic ballast is assumed to cause less temperature stress to the lamp than the conventional ballast. Also with regard to a maximum lamp wattage applied to the lamp, the electronic ballast gives a less maximum lamp wattage because of less lamp voltage and current than the conventional ballast, and is therefore assumed to lower the temperature stress. In this consequence, the electronic ballast is expected to extend lamp operating life.
Nevertheless, the electronic ballast is found to give only reduced lamp operating life. In fact, some lamps operated by the electronic ballast are found to have a reduced lamp operating life as less as one-half of that operated by the conventional ballast. Through investigations, the inventors have revealed that a local temperature rise occurs more significantly in an arc tube of the lamp when operated by the electronic ballast than by the conventional ballast. The investigations were focused on an arc luminescence as representative of the local temperature of the lamp. The arc luminescence was measured by operating the horizontally placed lamp, as shown in FIG. 13, and defined as a maximum luminescence at a point X of an arc developed between electrodes 2 of the lamp to have varying luminescence along the length of the arc, and is acknowledged to be in a generally direct proportion to the local temperature of the lamp. The investigation was carried out to measure points of equal arc luminescence by varying lamp wattage W.sub.LA and voltages V.sub.LA respectively with the use of the electronic ballast and the conventional ballast for three lamps, i.e., new one, old one just reaching end of the lamp life and exhibiting a maximum lamp voltage, and an intermediate one. The lamps utilized are 150W metal halide lamps sold under the name of HQI-TS 150W/NDL by OSRAM, Germany. Three levels of arc luminescence were selected for each lamp in order to obtain the equi-luminescence points 1, 2, and 3 of different levels with correspondingly differing lamp wattage and voltage, the arc luminescence level becomes greater in this order (3&gt;2&gt;1). Thus measured points were plotted, as shown in FIG. 13, to give lines N.sub.EW1 and N.sub.EW2, I.sub.NT1 and I.sub.NT2, and O.sub.LD1 and O.sub.LD2 for the new, intermediate, old lamps operated respectively by the electronic ballast and the conventional ballast, in which the solid lines and dotted lines denote the varying arc luminescence of the lamp operated, respectively by the electronic ballast and the conventional ballast. From FIG. 13, it is known that so far as the lamp voltage V.sub.LA is around the rated voltage V.sub.01, no substantial difference is seen between the lamp wattage required at the electronic ballast and the conventional ballast to give the same arc luminescence. But, after the lamp voltage increasing further beyond the rated voltage V.sub.01 as a result of the lamp having experienced a long period use, considerable differences are seen between the lamp wattage required to give the same arc luminescence of the levels 2 and 3, respectively. In other words, as the lamp voltage increases further beyond the rated lamp voltage, it is seen that, when the same lamps are operated by the electronic ballast and the conventional ballast to give the same lamp wattage, i.e., illumination level, the lamp operated by the electronic ballast shows the arc luminescence which is significantly greater than the lamp operated by the conventional ballast. This means that the electronic ballast brings about a significant local temperature rise in the lamp as compared to the conventional ballast when operating the lamp at the same lamp wattage. Such significant local temperature rise is thought attributable to the improved lamp power factor that the electronic ballast gives. That is, the electronic ballast requires less lamp current than the conventional ballast in order to give the same lamp wattage, and gives narrower arc than the conventional ballast when effecting the same average arc temperature. Consequently, it is assumed that, as the lamp voltage exceeds the rated voltage, the lamp current concentrates in the center of the arc to thereby remarkably increase the arc luminescence, or the maximum luminescence at the center of the arc. As the arc luminescence becomes greater with attendant local temperature rise in the lamp, the arc tube made of silica glass is exposed at its center to local heat concentration. When such local heat concentration becomes significant, the silica glass undergoes recrystallization to result a whitely turbid portion. Thus whitely turbid portion is thought to reflect light and heat from the arc on the other portion of the tube to thereby raise the overall temperature of the tube, which eventually deteriorates the lamp to such a level not capable of operating any further. As for the lamp filled with sodium, the local or overall temperature rise of the arc tube is thought to bring about leakage of sodium, thereby critically deteriorating the lamp.
In order to avoid the above problem for extending the lamp operating life, the inventors have proposed in the Japanese patent publication (KOKOKU) No. 576158 a method of operating the high pressure gas discharge lamp with the use of the electronic ballast having a lamp power factor of about 1.0. The method is characterized to vary the lamp wattage in accordance with a lamp equi-luminescence characteristic curve so as to keep arc luminescence of the arc at a constant level over the varying lamp voltage. The equi-luminescence characteristic curve is analogous to three solid line curves X.sub.1, X.sub.2, and X.sub.3 which are given in FIG. 13 by exploration of equal luminescence points, but is selected to represent the arc luminescence level which is equal to that produced at the rated lamp voltage. With this method, it is possible to eliminate undue increase in the arc luminescence, i.e., local temperature rise as the lamp is operated at the increasing lamp voltage beyond the rated lamp voltage, thereby extending the lamp operation life.
Although the above method is effective to extend the lamp life, it poses another problem that the lamp wattage is lowered sharply with attendance lowering of luminous flux when the lamp voltage increases beyond the rated voltage, as seen from the characteristic curves X.sub.1, X.sub.2, and X.sub.3 in FIG. 13.